[PATCH v13 06/13] x86/um: nommu: process/thread handling

Hajime Tazaki
2025-11-08T08:06:11Z

Since ptrace facility isn't used under !MMU of UML, there is different
code path to invoke processes/threads; there are no external process
used, and need to properly configure some of registers (fs segment
register for TLS, etc) on every context switch, etc.

Signals aren't delivered in non-ptrace syscall entry/leave so, we also
need to handle pending signal by ourselves.

ptrace related syscalls are not tested yet so, marked
arch_has_single_step() unsupported in !MMU environment.

Signed-off-by: Hajime Tazaki <thehajime@gmail.com>
Signed-off-by: Ricardo Koller <ricarkol@google.com>
---
arch/um/include/asm/ptrace-generic.h | 2 +-
arch/x86/um/Makefile | 3 +-
arch/x86/um/nommu/Makefile | 2 +-
arch/x86/um/nommu/entry_64.S | 2 ++
arch/x86/um/nommu/syscalls.h | 2 ++
arch/x86/um/nommu/syscalls_64.c | 50 ++++++++++++++++++++++++++++
6 files changed, 58 insertions(+), 3 deletions(-)
create mode 100644 arch/x86/um/nommu/syscalls_64.c

diff --git a/arch/um/include/asm/ptrace-generic.h b/arch/um/include/asm/ptrace-generic.h
index 62e9916078ec..5aa38fe6b2fb 100644
--- a/arch/um/include/asm/ptrace-generic.h
+++ b/arch/um/include/asm/ptrace-generic.h
@@ -14,7 +14,7 @@ struct pt_regs {
struct uml_pt_regs regs;
};

-#define arch_has_single_step() (1)
+#define arch_has_single_step() (IS_ENABLED(CONFIG_MMU))

#define EMPTY_REGS { .regs = EMPTY_UML_PT_REGS }

diff --git a/arch/x86/um/Makefile b/arch/x86/um/Makefile
index 39693807755a..98dc57afff83 100644
--- a/arch/x86/um/Makefile
+++ b/arch/x86/um/Makefile
@@ -26,7 +26,8 @@ subarch-y += ../kernel/sys_ia32.o

else

-obj-y += mem_64.o syscalls_64.o vdso/
+obj-y += mem_64.o vdso/
+obj-$(CONFIG_MMU) += syscalls_64.o

subarch-y = ../lib/csum-partial_64.o ../lib/memcpy_64.o \
../lib/memmove_64.o ../lib/memset_64.o
diff --git a/arch/x86/um/nommu/Makefile b/arch/x86/um/nommu/Makefile
index ebe47d4836f4..4018d9e0aba0 100644
--- a/arch/x86/um/nommu/Makefile
+++ b/arch/x86/um/nommu/Makefile
@@ -5,4 +5,4 @@ else
BITS := 64
endif

-obj-y = do_syscall_$(BITS).o entry_$(BITS).o os-Linux/
+obj-y = do_syscall_$(BITS).o entry_$(BITS).o syscalls_$(BITS).o os-Linux/
diff --git a/arch/x86/um/nommu/entry_64.S b/arch/x86/um/nommu/entry_64.S
index 485c578aae64..a58922fc81e5 100644
--- a/arch/x86/um/nommu/entry_64.S
+++ b/arch/x86/um/nommu/entry_64.S
@@ -86,6 +86,8 @@ END(__kernel_vsyscall)
*/
ENTRY(userspace)

+ /* set stack and pt_regs to the current task */
+ call arch_set_stack_to_current
/* clear direction flag to meet ABI */
cld
/* align the stack for x86_64 ABI */
diff --git a/arch/x86/um/nommu/syscalls.h b/arch/x86/um/nommu/syscalls.h
index a2433756b1fc..ce16bf8abd59 100644
--- a/arch/x86/um/nommu/syscalls.h
+++ b/arch/x86/um/nommu/syscalls.h
@@ -13,4 +13,6 @@
extern long current_top_of_stack;
extern long current_ptregs;

+void arch_set_stack_to_current(void);
+
#endif
diff --git a/arch/x86/um/nommu/syscalls_64.c b/arch/x86/um/nommu/syscalls_64.c
new file mode 100644
index 000000000000..d56027ebc651
--- /dev/null
+++ b/arch/x86/um/nommu/syscalls_64.c
@@ -0,0 +1,50 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2003 - 2007 Jeff Dike (jdike@{addtoit,linux.intel}.com)
+ * Copyright 2003 PathScale, Inc.
+ *
+ * Licensed under the GPL
+ */
+
+#include <linux/sched.h>
+#include <linux/sched/mm.h>
+#include <linux/syscalls.h>
+#include <linux/uaccess.h>
+#include <asm/prctl.h> /* XXX This should get the constants from libc */
+#include <registers.h>
+#include <os.h>
+#include "syscalls.h"
+
+void arch_set_stack_to_current(void)
+{
+ current_top_of_stack = task_top_of_stack(current);
+ current_ptregs = (long)task_pt_regs(current);
+}
+
+void arch_switch_to(struct task_struct *to)
+{
+ /*
+ * In !CONFIG_MMU, it doesn't ptrace thus,
+ * The FS_BASE registers are saved here.
+ */
+ current_top_of_stack = task_top_of_stack(to);
+ current_ptregs = (long)task_pt_regs(to);
+
+ if ((to->thread.regs.regs.gp[FS_BASE / sizeof(unsigned long)] == 0) ||
+ (to->mm == NULL))
+ return;
+
+ /* this changes the FS on every context switch */
+ arch_prctl(to, ARCH_SET_FS,
+ (void __user *) to->thread.regs.regs.gp[FS_BASE / sizeof(unsigned long)]);
+}
+
+SYSCALL_DEFINE6(mmap, unsigned long, addr, unsigned long, len,
+ unsigned long, prot, unsigned long, flags,
+ unsigned long, fd, unsigned long, off)
+{
+ if (off & ~PAGE_MASK)
+ return -EINVAL;
+
+ return ksys_mmap_pgoff(addr, len, prot, flags, fd, off >> PAGE_SHIFT);
+}
--
2.43.0